A new study links casual marijuana use with significant changes to parts of the brain.

Using marijuana a few times a week is enough to physically alter critical brain structures, according to a new study published Tuesday in The Journal of Neuroscience.

"Just casual use appears to create changes in the brain in areas you don't want to change," said Hans Breiter, a psychiatrist and mathematician at the Northwestern University Feinberg School of Medicine in Chicago, who led the new study.

There is actually very little research on the potential benefits and downsides of casual marijuana smoking — fewer than four times a week on average.

In his study, done in collaboration with researchers at Harvard University, scientists looked at the brains of 20 relatively light marijuana users and 20 people who did not use it at all. All 40 were college students in the Boston area.

The study found volume, shape and density changes in two crucial brain areas — the nucleus accumbens and the amygdala — involved with emotion and motivation and some types of mental illness. "This is a part of the brain you do not want to mess around with," Breiter said.

The more marijuana the students smoked, the more their brains differed from the non-users, the study found.

The brain continues to develop well into the 20s, and even into the 30s, said Breiter, who is concerned about the long-term impacts of marijuana use on the developing brain.

Staci Gruber, an associate professor of psychiatry at Harvard Medical School, who was not involved in the research, said Breiter's findings are consistent with her own, although she has focused on somewhat heavier users.

"There have been a growing number of studies that suggest that marijuana use in emerging adults is associated with differences in brain structure and cognitive abilities," said Gruber, also the director of Cognitive and Clinical Neuroimaging Core at McLean Hospital outside Boston. "I'm not saying (pot smoking) is analogous to shooting heroin or cocaine, but it's also not quite the benign substance people thought it was."

Responding to a study that found a decline in IQ points among people who used marijuana regularly, Francis Collins, the director of the National Institutes of Health, told USA TODAY recently that people should be more aware of these potential brain impacts.

"Perhaps it would be better if ... there was a little bit more recognition of that particular consequence," he said.

Gregory Gerdeman, a biologist and neuropharmacologist at Eckerd College in St. Petersburg, Fla., said he has no reason to doubt the new study's findings but worries generally about marijuana research funded by federal agencies, like the Office of National Drug Control Policy, which is charged with limiting drug use. (The research was supported by grants from the National Institutes of Health as well as the Office of National Drug Control Policy and Northwestern Medicine's Warren Wright Adolescent Center.)

"If you're getting money from the drug czar's office, that money's not going to continue if you don't end up publishing something that at least supports the general story of the danger of drug abuse," Gerdeman said.

He said it doesn't surprise him that heavy pot smoking might make it difficult for students to reach their intellectual potential. But still, he said, "if it were my child, even with this study, I'm more comfortable with young people having a casual marijuana habit than drinking regularly."